OFDM is a very effective technique for data communications in several environments because of its ability to reduce the negative effects of channel distortions such as selective fading and narrow band interference. (“OFDM for Wireless Multimedia Communications”; by R. van Nee, R. Prasad; Artech House Publishers; 2000; ISBN 0-89006-530-6. OFDM uses a multi-carrier transmission scheme (i.e., sub-channels or tones) for both synchronization and data transfer. Examples of communications systems using OFDM include IEEE 802.11a (IEEE Standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications and supplements (wireless LAN applications), DAB (“Digital Audio Broadcast, Guide to DAB Standards; Guidelines and Bibliography”, ETSI, TR 101 495 V1.1.1), DVB-T (“Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television”, ETSI, EN 200,744), HomePlug Powerline Alliance (HPA) Specification 1.0, HomePlug Powerline Alliance Specification AV1.1, and others. The above-listed references are incorporated herein by reference.
In a network of one or more OFDM transmitters, the channel is used to communicate between units and there are time segments when transmitter signals are present and times when there are no transmitter signals on the channel as illustrated FIG. 1. The time durations when transmitters occupy the channel vary depending on parameters such as the block size of the data being sent (payload data or control data). Segments of time when no transmitters occupy the channel have different durations that can vary based on situations such as: a single transmitter sending consecutive blocks of data with gaps between blocks, multiple transmitters contending for access to the channel and a channel idle condition. The lengths of these time segments are predictable (except for the situation where no units are communicating) and can be determined based on knowledge of the appropriate communications standard (e.g., 802.11a, DVB-T, etc.). Knowledge of the communications standard also provides information about the structure of transmitter signals (e.g., preamble structure, priority resolution structure, etc.). Knowing the structure of the signal, especially preset information such as transmission structure, provides data that can be used to estimate the quality of the received signal and the channel itself. Information gathered when no transmitters occupy the channel can be used to estimate channel noise. This approach could be used on the power up of the system or during operation. The information this process would result to may provide a viable input related to the following:                Allocation of pilot tones when necessary;        Detection of other narrow-band signals;        Detection of other wide-band systems;        Detection of beacons, etc.        
The quality of each sub-channel at any given time determines how well the overall system can transport data. A good quality sub-channel provides good synchronization information, which is then used to recover data correctly. A poor quality sub-channel could mean data loss due to errors in synchronization or unrecoverable errors in the data itself. Furthermore, time taken to evaluate sub-channel quality as part of the transmission process, time taken to retransmit data due to channel related errors or time taken to regularly distribute estimated channel quality all tend to reduce the overall data rate. Improvements in channel quality estimates used at the local receiver, without direct cooperation with any remote transceivers, would therefore improve the quality of communications (i.e., facilitate higher data rates and reduce error rates).
One of the first steps an OFDM receiver must perform in order to extract data from the channel, is to perform synchronization. Two types of synchronization are required: OFDM symbol boundary identification/timing and sub-carrier frequency/phase offset estimation/correction. FIG. 2 illustrates the blocks in a typical OFDM transceiver (PHY layer). The highlighted blocks in the following list are directly involved in synchronization:
TransmitterReceiverSerial data inputSerial data output[1] Coding (FEC,[15] Decoding (FEC,Encryption, etc.)Encryption, etc.)[2] Interleaving[14] De-interleaving[3] Mapping/[13] Demapping/Pilot insertionChannel correction[4] Modulation[12] Demodulation[5] iFFT[11] FFT[6] Cyclic extension,[10] Timing andwindowing &frequency sync & cyclicfilteringextension removal[7] DAC, RF Tx &[9] Coupler,coupler[8] PowerlineRF Rx & ADCChannel
The transmitter, in some implementations (e.g., 802.11a), inserts several fixed pilots (performed by block #3: Mapping/Pilot Insertion) on particular sub-channels to be used by the receivers channel estimator (sub-channel time and frequency estimations). While on other implementations (notably HPA) this block enables and disables sub-channels in cooperation with remote units (known as tone mapping). Part of the function of block #6 (Cyclic Extension, Windowing and Filtering) is to insert preset synchronization information before the transmission of the data block to be used by the receiver to estimate the timing and frequency offset of each OFDM symbol.
The two key receiver blocks, block #10 (Timing and Frequency Sync & Cyclic Extension Removal) and block #13 (Demapper/Channel Correction), correspond to block #6 and block #3 respectively on the transmit side and are responsible for, among other tasks, synchronization.
The other key receiver component is block #11 that performs FFT's on the channel signal. The output of this block contains amplitude and phase information at every OFDM carrier frequency.
Block #9, although not directly involved in synchronization and data recovery contains AGC circuits, which have an important role in acquiring good signals from the channel.
It is evident from the previous discussion that much of the mechanism needed to gather channel signal and channel noise data is already available within OFDM receivers. This invention focuses on improving both synchronization and data transfer of any OFDM system through the independent and continuous estimation of channel quality for use by the local receiver.